For purposes of clarity, the term “conventional engine” as used in the present application refers to an internal combustion engine wherein all four strokes of the well-known Otto cycle (the intake, compression, expansion (or power) and exhaust strokes) are contained in each piston/cylinder combination of the engine. Each stroke requires one half revolution of the crankshaft (180 degrees crank angle (CA)), and two full revolutions of the crankshaft (720 degrees CA) are required to complete the entire Otto cycle in each cylinder of a conventional engine.
Also, for purposes of clarity, the following definition is offered for the term “split-cycle engine” as may be applied to engines disclosed in the prior art and as referred to in the present application.
A split-cycle engine comprises:
a crankshaft rotatable about a crankshaft axis;
a compressor including a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft;
an expander including an expansion (power) piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft; and
a crossover passage interconnecting the compression and expansion cylinders, the crossover passage including at least a crossover expansion (XovrE) valve disposed therein, but more preferably including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween.
U.S. Pat. No. 6,543,225 granted Apr. 8, 2003 to Carmelo J. Scuderi (the Scuderi patent) and U.S. Pat. No. 6,952,923 granted Oct. 11, 2005 to David P. Branyon et al. (the Branyon patent) each contains an extensive discussion of split-cycle and similar type engines. In addition, the Scuderi and Branyon patents disclose details of prior versions of engines of which the present invention comprises a further development. Both the Scuderi patent and the Branyon patent are incorporated herein by reference in their entirety.
Referring to FIG. 1, a prior art exemplary embodiment of a split-cycle engine of the type similar to those described in the Branyon and Scuderi patents is shown generally by numeral 10. The split-cycle engine 10 replaces two adjacent cylinders of a conventional engine with a combination of one compression cylinder 12 and one expansion cylinder 14. A cylinder head 33 is typically disposed over an open end of the expansion and compression cylinders 12, 14 to cover and seal the cylinders.
The four strokes of the Otto cycle are “split” over the two cylinders 12 and 14 such that the compression cylinder 12, together with its associated compression piston 20, perform the intake and compression strokes (the compression cylinder 12 and piston 20 collectively being referred to as the compressor (12, 20)), and the expansion cylinder 14, together with its associated expansion piston 30, perform the expansion and exhaust strokes (the expansion cylinder 14 and piston 30 collectively being referred to as the expander (14, 30)). The Otto cycle is therefore completed in these two cylinders 12, 14 once per crankshaft 16 revolution (360 degrees CA) about crankshaft axis 17.
During the intake stroke, intake air is drawn into the compression cylinder 12 through an intake manifold (port) 19 disposed in the cylinder head 33. An inwardly opening (opening inward into the cylinder and toward the piston) poppet intake valve 18 controls fluid communication between the intake manifold 19 and the compression cylinder 12. The intake air is approximately at atmospheric pressure in the intake manifold.
During the compression stroke, the compression piston 20 pressurizes the air charge and, upon XovrC opening, drives the air charge into the crossover passage (or port) 22, which is typically disposed in the cylinder head 33. This means that the compression cylinder 12 and compression piston 20 are a source of high pressure gas to the crossover passage 22, which acts as the intake passage for the expansion cylinder 14. In some embodiments, two or more crossover passages 22 interconnect the compression cylinder 12 and the expansion cylinder 14.
The volumetric (or geometric) compression ratio of the compression cylinder 12 of split-cycle engine 10 (and for split-cycle engines in general) is herein referred to as the “compression ratio” of the split-cycle engine. The volumetric (or geometric) compression ratio of the expansion cylinder 14 of split-cycle engine 10 (and for split-cycle engines in general) is herein referred to as the “expansion ratio” of the split-cycle engine. The compression ratio of a cylinder is well known in the art as the ratio of the enclosed (or trapped) volume in the cylinder (including all recesses) when a piston reciprocating therein is at its bottom dead center (BDC) position to the enclosed volume (i.e., clearance volume) in the cylinder when the piston is at its top dead center (TDC) position. Specifically for split-cycle engines as defined herein, the volume of the crossover passage(s) is not included in the determination of the compression ratio of a compression cylinder. Also, specifically for split-cycle engines as defined herein, the volume of the crossover passage(s) is not included in the determination of the expansion ratio of an expansion cylinder.
Due to very high compression ratios (e.g., 20 to 1, 30 to 1, 40 to 1, or greater), an outwardly opening (opening outward away from the cylinder and piston) poppet crossover compression (XovrC) valve 24 at the crossover passage inlet 25 is used to control flow from the compression cylinder 12 into the crossover passage 22. Due to very high expansion ratios (e.g., 20 to 1, 30 to 1, 40 to 1, or greater), an outwardly opening poppet crossover expansion (XovrE) valve 26 at the outlet 27 of the crossover passage 22 controls flow from the crossover passage into the expansion cylinder 14. The actuation rates and phasing of the XovrC and XovrE valves 24, are timed to maintain pressure in the crossover passage 22 at a high minimum pressure (typically 20 bar absolute or higher, e.g., 40 to 50 bar, during full load operation) during all four strokes of the Otto cycle.
At least one fuel injector 28 injects fuel into the pressurized air at the exit end of the crossover passage 22 in correspondence with the XovrE valve 26 opening, which occurs shortly before expansion piston 30 reaches its top dead center position. At this time, the pressure ratio of the pressure in crossover passage 22 to the pressure in expansion cylinder 14 is high, due to the fact that the minimum pressure in the crossover passage is typically 20 bar absolute or higher at full engine load and the pressure in the expansion cylinder during the exhaust stroke is typically about one to two bar absolute. In other words, when XovrE valve 26 opens, the pressure in crossover passage 22 is substantially higher than the pressure in expansion cylinder 14 (typically in the order of 20 to 1 or greater at full engine load). This high pressure ratio causes initial flow of the air and/or fuel charge to flow into expansion cylinder 14 at high speeds. These high flow speeds can reach the speed of sound, which is referred to as sonic flow. The air/fuel charge usually enters the expansion cylinder 14 shortly after expansion piston 30 reaches its top dead center position (TDC), although it may begin entering slightly before TDC under some operating conditions. As piston 30 begins its descent from its top dead center position, and while the XovrE valve 26 is still open, spark plug 32, which includes a spark plug tip 39 that protrudes into cylinder 14, is fired to initiate combustion in the region around the spark plug tip 39. Combustion can be initiated while the expansion piston is between 1 and 30 degrees CA past its top dead center (TDC) position. More preferably, combustion can be initiated while the expansion piston is between 5 and 25 degrees CA past its top dead center (TDC) position. Most preferably, combustion can be initiated while the expansion piston is between 10 and 20 degrees CA past its top dead center (TDC) position. Additionally, combustion may be initiated through other ignition devices and/or methods, such as with glow plugs, microwave ignition devices or through compression ignition methods. The sonic flow of the air/fuel charge is particularly advantageous to split-cycle engine 10 because it causes a rapid combustion event, which enables the split-cycle engine 10 to maintain high combustion pressures even though ignition is initiated while the expansion piston 30 is descending from its top dead center position.
The XovrE valve 26 is closed after combustion is initiated but before the resulting combustion event can enter the crossover passage 22. The combustion event drives the expansion piston 30 downward in a power stroke.
During the exhaust stroke, exhaust gases are pumped out of the expansion cylinder 14 through exhaust port 35 disposed in cylinder head 33. An inwardly opening poppet exhaust valve 34, disposed in the inlet 31 of the exhaust port 35, controls fluid communication between the expansion cylinder 14 and the exhaust port 35.
Typically, in a naturally aspirated split-cycle engine such as that shown in FIG. 1, the compression cylinder displacement volume (Vd) required to intake a given charge (or mass) of air is larger than the required displacement volume of a cylinder of a conventional engine for the same charge of air. The compression cylinder of a naturally aspirated split-cycle engine must be made larger because during engine operation there is always a trapped compressed air mass present in the compression cylinder at the end of the compression stroke. Therefore, during the subsequent intake stroke immediately following the compression stroke, intake air cannot be drawn into the compression cylinder until the compression piston drops down far enough from top dead center so that the pressure of the trapped air mass is equal to atmospheric pressure. Thus, part of the volume swept by the compression cylinder during the intake stroke is not utilized for air intake. Hence, the compression cylinder must be made larger so that it has sufficient volume to draw a necessary amount of intake air during the remainder of the intake stroke. This increase in displaced volume decreases the power density of a typical naturally aspirated split-cycle engine, the power density (or specific power) being defined as the brake power per engine displacement, usually expressed as kilowatts/liter or horsepower/liter.
It is also known in the art of internal combustion engines to operate a conventional engine using the Miller cycle. The efficiency of an internal combustion engine is increased if the gas is expanded more during the expansion stroke than it is compressed during the compression stroke. In the Miller cycle of a conventional engine, this is typically accomplished by early or late inlet valve closing (IVC), which decreases the effective compression ratio relative to the expansion ratio. For example, if the inlet valve of a conventional engine is closed late (i.e., during the compression stroke that follows the intake stroke), a portion of the intake air that was drawn into the cylinder during the intake stroke is pushed back out of the cylinder through the intake port. The intake valve may be kept open during about the first 20 percent of the compression stroke. Therefore, actual compression only occurs in about the last 80 percent of the compression stroke.
Referring to FIG. 2A, an exemplary embodiment of a pressure vs. volume (PV) diagram of a naturally aspirated engine utilizing late IVC to effect Miller cycle operation is shown. Though this embodiment depicts a naturally aspirated engine, it is know that the same principles apply to turbocharged engines as well.
As shown in FIG. 2A, during the intake stroke of the piston from TDC to BDC, the cylinder pressure follows a constant pressure line from point 6 through point 1 and finally to point 5. During the initial portion of the subsequent compression stroke, while the intake valve is left open the cylinder pressure retraces the pressure line from point 5 back to point 1. Then, at point 1 the intake valve closes and the cylinder pressure increases from point 1 to point 2 during the remainder of the compression stroke. The volume swept by the piston along the path 1-5 is canceled by the volume swept along the path 5-1, and the effective compression ratio is the volume at point 1 divided by the volume at point 2 rather than the volume at point 5 divided by the volume at point 2 for the Otto cycle.
Referring to FIG. 2B, the same effect can be achieved in the Miller cycle by early inlet valve closing. In this case, the pressure remains constant during the intake stroke from point 6 to point 1. Then at point 1 the intake valve closes, and the pressure in the cylinder decreases from point 1 to point 7. During the subsequent compression stroke, the pressure increases from point 7 to point 1, canceling the previously traced path, and continues to point 2 during the remainder of the compression stroke. The net result is the same as late intake valve closing. That is, less than the entire piston stroke is effectively used for compression, thereby decreasing the effective compression ratio for increased efficiency while also decreasing the mass of charge air per cycle.
The increase in efficiency of the Miller cycle (typically 10 to 15 percent greater than the Otto cycle) is negatively offset by a decrease in indicated mean effective pressure (IMEP) and power density that is a result of a loss of charge air because only part of the total displaced volume in the cylinder is filled with charge air (i.e., displacement volume is sacrificed). Accordingly, to achieve the same amount of power as an Otto cycle engine, a Miller cycle engine typically must be made larger, or boosted, or boosted more aggressively.
Additionally, the later or earlier IVC occurs, the faster the piston is traveling and, therefore, the faster the air is flowing over the intake valve when it closes. This leads to significant pumping loses, which greatly reduces engine efficiency (i.e, brake specific fuel consumption (BSFC)) of conventional Miller cycle engines.